It is well known in the art to utilize a dual-mode electron gun within a travelling-wave tube (TWT) or a similar transient time tube. A travelling-wave tube is a broad-band, microwave tube which depends for its characteristics upon the interaction between the field of a wave propagated along a slow wave structure and a beam of electrons travelling with the wave. In this tube, the electrons in the beam travel with velocities slightly greater than that of the wave, and, on the average, are slowed down by the field of the wave. Thus, the loss in kinetic energy of the electrons appears as an increased energy conveyed by the field to the wave. The travelling-wave tube, therefore, acts as an amplifier or an oscillator.
In modern microwave radar, communications and electronic countermeasures systems, it is often desirable or necessary for the travelling wave tubes in these systems to operate at two different power levels. In one mode, the so-called low mode, the tube peak output power is at a level of P.sub.o watts, with a duty cycle of D.sub.u, which can be 100% in the continuous wave case. In the so-called high mode in a 10 dB up-pulse device, the peak output power is 10 P.sub.o watts whereas the duty cycle is reduced to 0.1 D.sub.u so as to keep the average power level from the device approximately the same in both modes. The numbers quoted here are examples only. Other combinations of duty cycle and tube output power levels may be preferable in certain systems including more than two discrete levels of power and duty cycle.
An example of a dual-gridded electron gun which utilizes a screen grid projected over only a peripheral portion of an electron emissive cathode surface in combination with a second control grid which extends substantially across the full emissive surface is shown in U.S. Pat. No. 3,903,450, issued Sept. 2, 1975, by R. A. Forbess, et al.
The flow of electron current from the smooth surface of a cathode around the screen grid produces a non-laminar flow of electrons which has been avoided by the creation of dimples or grooves in the surface of the cathode. It then becomes necessary to align the screen grid with the dimples so that the raised edges between the dimples coincide with the conductive elements within the grid. An example of a gridded electron gun utilizing a dimpled cathode may be found in U.S. Pat. No. 3,843,902, issued Oct. 22, 1974, by G. V. Miram, et al.
Other arrangements aimed at improving the laminar flow of electrons within a dual-mode electron gun may be found in U.S. Pat. No. 3,859,552, issued Jan. 7, 1975, by R. Hechtel and in U.S. Pat. No. 4,023,061, issued May 10, 1977, by A. E. Berwick, et al. Each of these devices incorporate a first partial inner grid formed by a circular pattern of conductive elements which are surrounded by a second partial outer grid in the pattern of an annular ring of conductive elements which surround the circular pattern of the first partial inner grid. The first grid having the inner circular pattern is crimped so that the circular pattern fits into and aligns with the annular pattern of the second outer grid. The crimp arrangement permits the two grids to be aligned within a single spherical surface. However, the crimp creates discontinuity which distorts the laminar flow of electron current.
A typical prior art device incorporating the features mentioned above includes a scalloped or dimpled cathode having a shadow grid and two control grids including a first grid having an inner circular pattern of conductive elements with crimped or kinked radial supports to fit into and spherically align with a second grid having an outer annular pattern of conductive elements. As mentioned above, the scalloped cathode is required to compensate against field distortion caused by the use of a third shadow grid. The shadow grid, on the other hand, is required to prevent the heating of the first and second grids by the electron beam emanating from the cathode. The typical prior art gun is difficult to align since the ridges formed in the scalloped cathode must align with the shadow grid and with the first and second grids. Further, the crimp or kink within the first grid causes a non-laminar flow of electrons.